This invention relates to spectrometers and in particular relates to suppressing undesired spectral components in the spectra obtained from the spectrometers. The invention has particularly but not exclusive application to FT-IR spectroscopy.
In for example an FT-IR spectrometer, infra-red or near infra-red radiation is directed from a source of such radiation towards a sample under investigation. Radiation transmitted by or reflected from the sample is received at a detector or receiver and the output of the detector is processed by a signal processor in order to obtain the spectral characteristics of the sample. In carrying out measurements it is first necessary to obtain what is known as a background measurement that is to say to the measure the background spectrum without a sample in place at the sample station. Subsequently measurements are made with the sample in place and the desired sample spectrum is obtained from the ratio of the measurement obtained with the sample in place to the background measurement.
A significant proportion of Spectroscopy performed in the mid infra-red range is concerned with spectra of solids and liquids which have absorbtion bandwidths in the tens of wave numbers. As a consequence, many of the measurements are made at, for example, 4 cmxe2x88x921 or 2 cmxe2x88x921 resolution. At such moderate resolutions water vapour having line widths closer to 0.1 cmxe2x88x921 is strongly under resolved and bands that might otherwise be saturated with absorbance well in excess of 1.0 are broadened to the point where they show relatively low peak absorbance. Two major consequences result from this under resolution: The absorbance becomes substantially non-linear with concentration causing the absorbance spectrum shape to become a strong function of concentration and in additional the lineshapes become entirely dominated by the instrument lineshape function making the spectra have a different character according to the instrument type and set up. More significantly since the instrument lineshape function may well be influenced by the sample or sampling accessory through vignetting another beamed geometry disturbances the water vapour spectrum in the sample spectrum may not entirely resemble the nominally similar spectrum in the background. That is to say the effects do not cancel out when the sample spectra and the background spectra are ratioed. The overall result is that it is extremely difficult to subtract out the effects of water vapour consistently by any means of linear spectrum differencing typically employed. A similar problem exists with other unwanted components such as those of carbon dioxide.
A further aspect of the problem is related to the overlap of features in the sample spectrum with features in the water vapour spectrum. Even when an exact spectrum of water vapour measured under the current sampling conditions can be generated independently by some means, it is very difficult to estimate precisely the proportion of water vapour spectrum that must be subtracted out from the detected spectra especially by automatic algorithmic approaches.
The present invention is concerned with a novel technique for subtracting out from the measured sample spectrum undesired components such as those arising from water vapour and carbon dioxide.
According to the present invention there is provided a spectrometer which comprises a source of analysing radiation, a detector for detecting radiation transmitted through or reflected from a sample under investigation, and a processing means for processing the output of the detector to produce spectral data relating to a sample under investigation, wherein the processor is arranged, in order to suppress the effects of unrequired components such as water vapour or carbon dioxide in the spectral data, to acquire reference data representing high resolution spectra of the unwanted component or components, to modify said data so that its resolution simulates that of the spectrometer, to filter said data so as to allow for pertutbing effects in the sample spectrum and to subtract the resulting data from the measured sample spectrum in order to provide a corrected output data. The acquired data may represent said high resolution spectra at a plurality of temperatures. The modification may comprise processing said reference data so as to broaden the resolution to that which matches the sample spectral data being measured. The broadening may be carried out by a convolution technique in which the reference data are convolved with a computed line shape function appropriate to the spectrometer. These steps may be repeated for different reference data representing different concentrations of unwanted component or components.
The processor may also be arranged to generate perturbed versions of the broadened reference data to take into account variations in at least some operating parameters such as temperature and optical line width.
The filtering may include creating a filter for filtering the spectra to emphasise higher resolution parts of the unrequited components. The filter may comprise a band pass filter. Filtering may also include carrying out a least square fit of the filtered sample spectral data to the filtered unwanted component spectra. Additionally the processor may iterate the least square fit to remove those parts of the spectrum which have a poor fit.
The processor may be arranged to use the resulting co-efficients to compute an unfiltered unwanted component spectrum. This is then subtracted from the measured sample spectrum in order to obtain a corrected sample spectrum.